Risk assessment-based design method for deep complex formation wellbore structure

ABSTRACT

A risk assessment-based design method for a deep complex formation wellbore structure includes: (1) preliminarily determining casing layers and setting depths; (2) calculating to obtain the risk coefficients of each layer of casing; (3) analyzing and coordinating, according to the principle that a shallow casing shares more risks and a deep casing shares less risks, the risks of each layer of casing: determining whether the risk coefficients of each layer of casing are greater than a safety threshold value K; checking the setting depth: if the safety coefficient of an ith-layer casing satisfies R Ni &gt;K, selecting a casing layer with the minimum safety coefficient from upper casing layers, and deepening the setting depth h of the casing layer; and (4) repeating the steps (2) to (3) until the casing risk coefficients of each layer of casing are less than the safety threshold value K.

FIELD OF THE INVENTION

The present invention relates to a risk assessment-based design methodfor a deep complex formation wellbore structure, and belongs to thetechnical field of oil and gas drilling.

BACKGROUND OF THE INVENTION

Wellbore structure design is one of the important contents of drillingengineering design, and the rationality of a wellbore structure designscheme directly affects safe and efficient implementation of drillingand completion construction.

There are many factors affecting the wellbore structure design, mainlyincluding: safe density window of drilling fluid, a geological settingposition, a geological target, drilling cost and the like. Through theresearch and development of domestic and foreign experts and scholars,basic methods for wellbore structure designs of bottom to top, top tobottom, middle to two sides and mixed designs are gradually formed,which provides a guarantee for safe and efficient construction ofdrilling and completion in different regions, reservoirs and workingconditions. However, as oil and gas exploration gradually moves into thefields of deep water and deep formation, the complexity and uncertaintyof a deep formation bring greater challenges to the wellbore structuredesign: for example, the prediction accuracy of formation pressure is animportant guarantee for the rationality of the wellbore structuredesign, but the prediction of the formation pressure before drilling atpresent has the problems of high upper formation prediction accuracy andlow deep formation prediction accuracy, which results in that, in thewellbore structure design and construction processes, a shallowformation has redundancy in wellbore structure safety, while the deepformation often has risks in wellbore structure safety due to largeprediction error of the formation pressure before drilling, and downholecomplex situations often occur because of an imperfect wellborestructure design in a construction process. On the other hand, thedesign coefficient of a common wellbore structure at present is a valuerange recommended according to a drilling design manual and regionalcharacteristics. In design, only one fixed value can be selected withinthe value range for designing according to experience and regionaldrilling data. As a result, the design coefficient of the whole well isa single numerical value, and if the design coefficient is selected toolarge, there may be redundancy for the shallow formation; and if thedesign coefficient is selected too small, it may be insufficient for thedeep formation.

Therefore, it is necessary to develop a wellbore structure design methodwith the coordination of risks of each layer of casing based on theconcept of risk assessment aiming at the characteristics of deep complexformation drilling and considering the formation prediction error ofdifferent well depths and the risk bearing capacity of each layer ofwellbore structure.

SUMMARY OF THE INVENTION

Aiming at the defects in the prior art, the present invention disclosesa risk assessment-based design method for a deep complex formationwellbore structure.

SUMMARY OF THE INVENTION

According to the present invention, in view of the characteristics ofinsufficient understanding of deep formation information and frequentoccurrence of downhole complex situations, the risk of a deep wellborestructure is moderately moved upwards by coordinating the risks borne bythe casings of all layers, more design space is provided for the casinglayers and the setting depth of a deep formation, the comprehensive riskof the whole wellbore structure is reduced to the maximum extent, and aguarantee for safe and efficient drilling is provided.

The specific technical scheme of the present invention is as follows:

A risk assessment-based design method for a deep complex formationwellbore structure, including:

(1) preliminarily determining casing layers and setting depths;

(2) calculating risk coefficients of each layer of casing;

(3) analyzing and coordinating, according to a principle that a shallowcasing shares more risks and a deep casing shares less risks, the risksof each layer of casing:

determining whether the risk coefficients of each layer of casing aregreater than a safety threshold value K, and setting the safetythreshold value K according to the safety requirement of a target well;

checking the setting depths: if the safety coefficient of an ith-layercasing satisfies R_(Ni)>K, selecting a casing layer with the minimumsafety coefficient from upper casing layers, and deepening the settingdepth h of the casing layer; and

(4) repeating (2) to (3) until the risk coefficients of each layer ofcasing are less than the safety threshold value K.

According to the present invention, preferably, the method forpreliminarily determining the casing layers and the setting depths in(1) at least includes:

(1-1) determining a geological setting position; namely, determining asetting horizon according to geological data, wherein the “determining asetting point” here is a necessary link in the wellbore structuredesign, that is, a blocked horizon is determined by analyzing thegeological data and regional drilling data according to the horizon andthe depth at which the geology is complex and a downhole accident occurseasily, so that a layer of casing must be designed correspondingly forblocking the setting position at the depth (horizon) in actualconstruction;

(1-2) preliminarily determining a safety pressure window, wherein thesafety pressure window is preliminarily determined according toprediction results of formation pore pressure, formation fracturepressure and formation collapse pressure before drilling and a pressurebalance relationship of an open hole section; and

(1-3) preliminarily determining the casing layers and the setting depthsthereof by a conventional “top to bottom” design method according to theresults of (1-1) and (1-2) and a regional wellbore structure designcoefficient. The present invention mainly focuses on the prominentproblem of the drilling risks of the deep formation in a deep drillingprocess; therefore, a “top to bottom” method is adopted, which makeseach layer of casing go to the deepest and maximizes a design window ofthe deep formation.

According to the present invention, preferably, the method forcalculating to obtain the risk coefficient of each layer of casing in(2) is as follows:

(2-1) probabilistic distribution of formation pressure

the prediction error ΔP_(i) of the formation pressure P_(i) is afunction of the well depth H:

ΔP _(i) =f(H)ϵ[P _(i0) ,P _(i1)]  (1)

in formula (1), P_(i0) is the lower limit value of the error, P_(i1) isthe upper limit value of the error, and i represents the type of theformation pressure;

the characteristic that the prediction error of the formation pressurebefore drilling is increased along with the increase of the well depthis introduced into the method of the present invention, and theprediction error of the formation pressure is given by others beforedesign and is subjected to probabilistic distribution in the presentinvention,

wherein the probabilistic distribution of the prediction error of theformation pressure satisfies the following rule:

$\begin{matrix}{{f\left( P_{i} \right)} = {\frac{1}{\sqrt{2\pi}\sigma_{p_{i}}}{\exp\left( {- \frac{\left( {P_{i} - \frac{P_{i1} + P_{i0}}{2}} \right)^{2}}{2\sigma_{p_{i}}^{2}}} \right)}}} & (2)\end{matrix}$

in formula (2), σ_(P) _(i) the standard deviation of P_(i) and isselected according to the prediction accuracy, and the value range is(0, 1); and in the present invention, the risk coefficient is calculatedby the cumulative probability of the formation pressure and thecumulative probability of other wellbore structure design coefficients,where σ_(P) _(i) determines the “width” of a probabilistic distributionfunction, namely, the width of the upper and lower limits of aprediction function. The wider the probabilistic distribution functionis, the more likely a real value falls into a predicted interval, thatis to say, the higher the prediction accuracy is, but a large predictionrange is not conducive to design.

According to the present invention, a specific error does not need to beobtained, and the prediction accuracy of the function is controlled byselecting different σ_(P) _(i) values. For example:

for a shallow formation with high prediction accuracy of the formationpressure, in order to increase the design window of a wellborestructure, the width of the upper and lower limits of the predictionfunction can be reduced moderately, and σ_(P) _(i) selected between 0.4and 0.6;

and for the deep formation with low prediction accuracy of the formationpressure, in order to reduce the risk of the wellbore structure, thewidth of the upper and lower limits of the prediction function can beincreased moderately, and σ_(P) _(i) selected between 0.6 and 0.8.

The cumulative probability corresponding to the predicted value P_(i) ofthe formation pressure is:

$\begin{matrix}{{P\left( P_{i} \right)} = {\int_{- \infty}^{P_{i}}{\frac{1}{{\sigma_{p}}_{i}\sqrt{2\pi}}e^{- \frac{{({P_{i} - \frac{P_{i1} + P_{i0}}{2}})}^{2}}{2\sigma_{p_{i}}^{2}}}{dP}_{i}}}} & (3)\end{matrix}$

for the formation pore pressure, the prediction error is ΔP_(p)ϵ[P_(p0),P_(p1)], and for the formation fracture pressure, the prediction erroris ΔP_(f)ϵ[P_(f0), P_(f1)];

(2-2) probabilistic distribution of wellbore structure designcoefficient

if the value range of the wellbore structure design coefficient K is[K₀, K₁], then the probabilistic distribution formula thereof is asfollows:

$\begin{matrix}{{f(K)} = {\frac{1}{\sqrt{2\pi}\sigma_{K}}{\exp\left( {- \frac{\left( {K - \frac{K_{1} + K_{0}}{2}} \right)^{2}}{2\sigma_{K}^{2}}} \right)}}} & (4)\end{matrix}$

in formula (4), σ_(K) is the standard deviation of K and is actuallyselected according to the drilling of a region where a target well islocated, and the value range is (0, 1);

if the occurrence frequency of a downhole engineering risk in a regionaldrilling practice is low, a relatively small σ_(K) value can be selectedwith regard to a shallow wellbore structure design coefficient; if theoccurrence frequency of the downhole engineering risk in the regionaldrilling practice is high, a relatively large σ_(K) value can beselected with regard to a deep wellbore structure design coefficient;for example: for the shallow formation, σ_(K) is selected between 0.4and 0.6; for the deep formation, σ_(K) is selected between 0.6 and 0.8;

a credibility J is set to obtain the distribution interval of eachdesign coefficient K as [f₀(K), f_(n)(K)]; in the distribution interval,the cumulative probability corresponding to the design coefficientf_(i)(K) is:

$\begin{matrix}{{P\left( {f_{i}(K)} \right)} = {\int_{- \infty}^{f_{i}{(K)}}{\frac{1}{\sigma_{K}\sqrt{2\pi}}e^{- \frac{{({{f_{i}{(K)}} - \frac{{f_{1}{(K)}} + {f_{0}{(K)}}}{2}})}^{2}}{2\sigma_{K}^{2}}}{d\left( {f_{i}(K)} \right)}}}} & (5)\end{matrix}$

the distribution intervals of kick tolerance S_(k), formation fracturepressure safety factor S_(f), additional drilling fluid density Δρ andsuction pressure factor S_(b) are respectively expressed as:[f₀(S_(k)),f_(n)(S_(k))], [f₀(S_(f)), f_(n)(S_(f))], [f₀(Δφ,f_(n)(Δρ)]and [f₀(S_(b)),f_(n)(S_(b))];

according to the present invention, preferably, the value of thecredibility J is 70%^(˜)95%;

at present, a common wellbore structure design coefficient is a valuerange recommended according to a drilling design manual and regionalcharacteristics, and a fixed value is selected from the value range fordesigning; according to the present invention, a probability statisticalmethod is adopted, regional well structure design coefficients aresubjected to probabilistic distribution, and different designcoefficients are selected with regard to the risk bearing capacity ofeach casing layer;

(2-3) downhole engineering risk calculation for an Nth-layer casing atthe well depth H

the downhole engineering risk R(H) at the well depth H is calculatedaccording to the pressure balance relationship:

$\begin{matrix}{{{kick}\mspace{14mu} {risk}\text{:}}\mspace{14mu}} & (6) \\{{R_{JY}(H)} = {{m\left\lbrack {1 - {P\left( {P_{p}(H)} \right)}} \right\rbrack} \times \left\lbrack {1 - {P\left( {f_{n}\left( S_{b} \right)} \right)}} \right\rbrack \times \left\lbrack {1 - {P\left( {f_{n}\left( {\Delta \rho} \right)} \right)}} \right\rbrack}} & \; \\{{where},{m = \left\{ \begin{matrix}0 & {\rho_{m} > {{P_{p}(H)} + {f_{n}\left( S_{b} \right)} + {f_{n}\left( {\Delta \rho} \right)}}} \\1 & {\rho_{m} \leq {{P_{p}(H)} + {f_{n}\left( S_{b} \right)} + {f_{n}\left( {\Delta \rho} \right)}}}\end{matrix} \right.}} & \;\end{matrix}$

risk of lost circulation:

$\begin{matrix}{{R_{JL}(H)} = {m \times {P\left( {P_{f0}(H)} \right)} \times \left\lbrack {1 - {P\left( {f_{n}\left( S_{k} \right)} \right)}} \right\rbrack \times \left\lbrack {1 - {P\left( {f_{n}\left( S_{f} \right)} \right)}} \right\rbrack}} & (7) \\{{where},{m = \left\{ \begin{matrix}0 & {\rho_{m} < {{{f_{n}\left( S_{k} \right)} \times \frac{H}{H_{n - 1}}} + {f_{n}\left( S_{f} \right)} + {P_{f0}(H)}}} \\1 & {\rho_{m} \geq {{{f_{n}\left( S_{k} \right)} \times \frac{H}{H_{n - 1}}} + {f_{n}\left( S_{f} \right)} + {P_{f0}(H)}}}\end{matrix} \right.}} & \;\end{matrix}$

in formulas (6) and (7), ρ_(m) is the equivalent density of drillingfluid, and H_(n-1) is the depth of the last casing shoe;

(2-4) determination of risk coefficients of each layer of casing

the downhole engineering risks at the well depth H calculated in (2-3)are integrated within the range of the layer of casing to obtain theoverall risk coefficient R_(N) of the Nth-layer casing

R _(N)=∫_(H) _(n) ^(H) ^(m) (R _(JY)(H)+R _(JL)(H)dH  (8)

in formula (8), H_(n) is the minimum depth of the Nth-layer casing; andH_(m) is the maximum depth of the Nth-layer casing.

The present invention has the technical advantages that:

According to the present invention, the above defects can be overcome byperforming probabilistic distribution on each of the design coefficientsand the prediction errors of the formation pressure and selecting theformation pressure prediction values and the design coefficients ofdifferent accuracy for different depths. Meanwhile, the risk coefficientof each layer of casing further can be calculated on that basis, therisks borne by each layer of casing are coordinated, and the overallwellbore structure risk is comprehensively reduced, so that the presentinvention has great advantages for the wellbore structure design of adeep well complex formation. According to the present invention, thewellbore structure design scheme that each layer of casing shares therisks based on the principle that the shallow casing shares more risksand the deep casing shares less risks is realized, which greatly reducesthe safety risk caused by the wellbore structure during the drillingprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a specific design contrast diagram for a wellbore structure inan embodiment of the present invention;

FIG. 2 is a flowchart of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A specific implementation mode is introduced by taking a well A as anexample. The design well depth is 6500 m; the kick tolerance S_(k)=0.05g/cm³, the formation fracture pressure safety factor S_(f)=0.04 g/cm³,the additional drilling fluid density Δρ=0.05 g/cm³ and the suctionpressure coefficient S_(b)=0.04 g/cm³. A formation pressure profile isas shown in FIG. 1.

A wellbore structure scheme of the well is preliminarily determined byadopting a top to bottom method according to (1) to (3) of the presentinvention.

In (2), the error cumulative probability formulas of the formation porepressure and the formation fracture pressure are respectively obtainedas follows by selecting the standard deviation of the formation pressureprediction error σ_(P) _(i) =0.6:

formation pore pressure:

${P\left( P_{pi} \right)} = {{\int_{- \infty}^{P_{pi}}{\frac{1}{\sigma \sqrt{2\pi}}e^{- \frac{{({P_{pi} - \frac{P_{{pi}\; 1} + P_{{pi}\; 0}}{2}})}^{2}}{2\sigma^{2}}}dP_{pi}}} = {\int_{- \infty}^{P_{pi}}{\frac{5}{3\sqrt{2\pi}}e^{- \frac{25{({P_{pi} - \frac{P_{{pi}\; 1} + P_{{pi}\; 0}}{2}})}^{2}}{18}}dP_{pi}}}}$

formation fracture pressure:

${P\left( P_{fi} \right)} = {{\int_{- \infty}^{P_{fi}}{\frac{1}{\sigma \sqrt{2\pi}}e^{- \frac{{({P_{fi} - \frac{P_{{fi}\; 1} + P_{{fi}\; 0}}{2}})}^{2}}{2\sigma^{2}}}dP_{fi}}} = {\int_{- \infty}^{P_{fi}}{\frac{5}{3\sqrt{2\pi}}e^{- \frac{25{({P_{fi} - \frac{P_{{fi}\; 1} + P_{{fi}\; 0}}{2}})}^{2}}{18}}dP_{fi}}}}$

According to the drilling experience of an adjoining well in the region,kick and lost circulation easily occur in the downhole with the depthinterval of 4000 to 5000 m, so that the wellbore structure designcoefficient σ_(P) _(i) =0.7 is selected for the depth greater than 4000m, and σ_(P) _(i) =0.5 is selected for other depths. The credibilityJ=90% is set to obtain the distribution intervals and the cumulativeprobability calculation formulas of each of the coefficients as follows:

Kick tolerance: the distribution interval is

$\left\{ \begin{matrix}\left\lbrack {{{0.0}4},{{0.0}6}} \right\rbrack & {H < {4000\mspace{14mu} m}} \\\left\lbrack {{{0.0}36},\ {{0.0}64}} \right\rbrack & {H \geq {4000\mspace{14mu} m}}\end{matrix} \right.\quad$

the cumulative probability formula is

${P\left( {f_{i}\left( S_{K} \right)} \right)} = \left\{ \begin{matrix}{\int_{- \infty}^{f_{i}{(S_{K})}}{\sqrt{\frac{2}{\pi}}e^{{- 2}{({{f_{i}{(S_{K})}} + 0.05})}^{2}}{d\left( {f_{i}\left( S_{K} \right)} \right)}}} & {H < {4000\mspace{14mu} m}} \\{\int_{- \infty}^{f_{i}{(S_{K})}}{\sqrt{\frac{50}{49\pi}}e^{- \frac{50{({{f_{i}{(S_{K})}} + 0.05})}^{2}}{49}}{d\left( {f_{i}\left( S_{K} \right)} \right)}}} & {H \geq {4000\mspace{14mu} m}}\end{matrix} \right.$

Formation fracture pressure safety factor: the distribution interval is

$\left\{ \begin{matrix}\left\lbrack {{{0.0}32},{{0.0}48}} \right\rbrack & {H < {4000\mspace{14mu} m}} \\\left\lbrack {{{0.0}3},\ {{0.0}5}} \right\rbrack & {H \geq {4000\mspace{14mu} m}}\end{matrix} \right.\quad$

the cumulative probability formula is

${P\left( {f_{i}\left( S_{f} \right)} \right)} = \left\{ \begin{matrix}{\int_{- \infty}^{f_{i}{(S_{f})}}{\sqrt{\frac{2}{\pi}}e^{{- 2}{({{f_{i}{(S_{f})}} + 0.04})}^{2}}{d\left( {f_{i}\left( S_{f} \right)} \right)}}} & {H < {4000\mspace{14mu} m}} \\{\int_{- \infty}^{f_{i}{(S_{f})}}{\sqrt{\frac{50}{49\pi}}e^{- \frac{50{({{f_{i}{(S_{f})}} + 0.04})}^{2}}{49}}{d\left( {f_{i}\left( S_{f} \right)} \right)}}} & {H \geq {4000\mspace{14mu} m}}\end{matrix} \right.$

Additional drilling fluid density: the distribution interval is

$\left\{ \begin{matrix}\left\lbrack {{{0.0}4},{{0.0}6}} \right\rbrack & {H < {4000\mspace{14mu} m}} \\\left\lbrack {{{0.0}36},\ {{0.0}64}} \right\rbrack & {H \geq {4000\mspace{14mu} m}}\end{matrix} \right.\quad$

the cumulative probability formula is

${P\left( {f_{i}\left( S_{K} \right)} \right)} = \left\{ \begin{matrix}{\int_{- \infty}^{f_{i}{(S_{K})}}{\sqrt{\frac{2}{\pi}}e^{{- 2}{({{f_{i}{(S_{K})}} + 0.05})}^{2}}{d\left( {f_{i}\left( S_{K} \right)} \right)}}} & {H < {4000\mspace{14mu} m}} \\{\int_{- \infty}^{f_{i}{(S_{K})}}{\sqrt{\frac{50}{49\pi}}e^{- \frac{50{({{f_{i}{(S_{K})}} + 0.05})}^{2}}{49}}{d\left( {f_{i}\left( S_{K} \right)} \right)}}} & {H \geq {4000\mspace{14mu} m}}\end{matrix} \right.$

Suction pressure coefficient: the distribution interval is

$\left\{ \begin{matrix}\left\lbrack {{{0.0}32},{{0.0}48}} \right\rbrack & {H < {4000\mspace{14mu} m}} \\\left\lbrack {{{0.0}3},{{0.0}5}} \right\rbrack & {H \geq {4000\mspace{14mu} m}}\end{matrix} \right.\quad$

The cumulative probability formula is

${{P\left( {f_{i}\left( S_{f} \right)} \right)} = \left\{ \begin{matrix}{\int_{- \infty}^{f_{i}{(S_{f})}}{\sqrt{\frac{2}{\pi}}e^{{- 2}{({{f_{i}{(S_{f})}} + 0.04})}^{2}}{d\left( {f_{i}\left( S_{f} \right)} \right)}}} & {H < {4000\mspace{14mu} m}} \\{\int_{- \infty}^{f_{i}{(S_{f})}}{\sqrt{\frac{50}{49\pi}}e^{- \frac{50{({{f_{i}{(S_{f})}} + 0.04})}^{2}}{49}}{d\left( {f_{i}\left( S_{f} \right)} \right)}}} & {H \geq {4000\mspace{14mu} m}}\end{matrix} \right.}.$

According to (2) to (3) in the present invention, in the embodiment,there are five layers of casings in total, and the downhole engineeringrisks of each layer of casing at different well depths are respectivelycalculated:

a first-layer casing: the kick risk R_(JY)=0; the lost circulation riskR_(JL)=0;

a second-layer casing: the kick risk

$R_{JY} = \left\{ {\begin{matrix}0 & {H < {2000\mspace{14mu} m}} \\{{9 \times 10^{- 6}x^{2}} - {{0.0}35x} + {3{5.2}79}} & {H \geq {2000\mspace{14mu} m}}\end{matrix};} \right.$

the lost circulation risk R_(JL)=0;

a third-layer casing: the kick risk

$R_{JY} = \left\{ {\begin{matrix}0 & {H < {3500\mspace{14mu} m}} \\{{2 \times 10^{- 6}x^{2}} - {0.0127x} - 23.657} & {H \geq {3500\mspace{14mu} m}}\end{matrix};} \right.$

the lost circulation risk

$R_{JL} = \left\{ {\begin{matrix}{{2 \times 10^{- 6}x^{2}} - {0.0091x} + 10.052} & {H < {2150\mspace{14mu} m}} \\0 & {H \geq {2150\mspace{14mu} m}}\end{matrix};} \right.$

a fourth-layer casing: the kick risk

$R_{JY} = \left\{ {\begin{matrix}0 & {H < {5150\mspace{14mu} m}} \\{{2 \times 10^{- 6}x^{2}} - {0.0217x} + 56.027} & {H \geq {5150\mspace{14mu} m}}\end{matrix};} \right.$

the lost circulation risk

$R_{JL} = \left\{ {\begin{matrix}{{{- 9} \times 10^{- 7}x^{2}} - {0.0067x} - 11.819} & {H < {3700\mspace{14mu} m}} \\0 & {H \geq {3700\mspace{14mu} m}}\end{matrix};} \right.$

a fifth-layer casing: the kick risk

$R_{JY} = \left\{ {\begin{matrix}0 & {H < {6380\mspace{14mu} m}} \\{{2 \times 10^{- 6}x^{2}} - {0.0255x} + 81.718} & {H \geq {6380\mspace{14mu} m}}\end{matrix};} \right.$

the lost circulation risk

$R_{JL} = \left\{ {\begin{matrix}{{{- 5} \times 10^{- 7}x^{2}} + {0.0055x} - 15.312} & {H < {5400\mspace{14mu} m}} \\0 & {H \geq {5400\mspace{14mu} m}}\end{matrix}.} \right.$

According to (2) to (4) in the present invention, the overall riskcoefficient of each layer of casing is obtained:

R₁=0; R₂=∫₃₀₀ ²¹⁰⁰ (R_(JY)(H)+R_(JL)(H)dH=0.532; R₃=∫₂₁₀₀ ³⁶⁰⁰(R_(JY)(H)+R_(L)(H)dH=0.483;

R₄=∫₃₆₀₀ ⁵¹⁰⁰(R_(JY)(H)+R_(JL)(H)dH=0.447; R₅=∫₅₁₀₀⁶⁵⁰⁰(R_(JY)(H)+R_(JL)(H)dH=0.408.

According to (3) to (4) in the present invention:

{circle around (1)}: a safety threshold value K=0.5 is set according toactual conditions, wherein the overall risk coefficient of thesecond-layer casing is greater than the value;

{circle around (2)}: the setting depth of the first-layer casing isincreased by 50 m;

{circle around (3)}: if the safety coefficient of the ith-layer casingsatisfies R_(Ni)>K, the casing layer with the minimum safety coefficientis selected from upper casing layers, and the setting depth h of thecasing layer is deepened; and

{circle around (4)}: until the risk coefficient of each layer of casingis less than the safety threshold value K.

In order to reflect the technical advantages of the present invention, acomparison is made between embodiments of the present invention andcomparative examples, wherein the comparative example described in Table1 refers to a comparative technical scheme formed according to (1) to(2) of the present invention.

TABLE 1 Comparative Example The Present Embodiment Drilling DrillingCasing Fluid Casing Fluid Casing Setting Density Risk Setting DensityRisk Layer Depth (g/cm³) Factor Depth (g/cm³) Factor 1 300 m 1.17 0 350m 1.18 0.035 2 2100 m 1.35 0.532 2125 m 1.37 0.0486 3 3600 m 1.68 0.4833635 m 1.72 0.043 4 5100 m 2.07 0.447 5110 m 2.23 0.0421 5 6500 m 2.630.408 6500 m 2.62 0.0413In combination with Table 1 and FIG. 1, it can be seen that, after theprocessing and design of the method disclosed by the present invention,the risks of the five layers of casings are all less than the safetythreshold value K=0.5, the casing setting depth of the shallow formationis deeper, and the depth of the open hole section of the deep formation(the setting depths of the fourth and fifth layers of casings) isreduced, which facilitates the reduction of downhole risk of the deepformation drilling, transfers the risk of a deep casing layer to ashallow casing layer, and reduces the overall risk.

What is claimed is:
 1. A risk assessment-based design method for a deepcomplex formation wellbore structure, comprising: (1) preliminarilydetermining casing layers and a setting depth; (2) calculating riskcoefficients of each layer of casing; (3) analyzing and coordinating,according to a principle that a shallow casing shares more risks and adeep casing shares less risks, the risks of each layer of casing:determining whether the risk coefficients of each layer of casing aregreater than a safety threshold value K; checking the setting depths: ifa safety coefficient of an ith-layer casing satisfies R_(Ni)>K,selecting a casing layer with the minimum safety coefficient from uppercasing layers, and deepening the setting depth h of the casing layer;and (4) repeating (2) to (3) until the risk coefficients of each layerof casing are less than the safety threshold value K.
 2. The riskassessment-based design method for the deep complex formation wellborestructure according to claim 1, wherein the method for preliminarilydetermining the casing layers and the running depths in (1) at leastcomprises: (1-1) determining a geological setting position; (1-2)preliminarily determining a safety pressure window, wherein the safetypressure window is preliminarily determined according to predictionresults of formation pore pressure, formation fracture pressure andformation collapse pressure before drilling and a pressure balancerelationship of an open hole section; and (1-3) preliminarilydetermining the casing layers and the setting depths thereof by aconventional “top to bottom” design method according to the results of(1-1) and (1-2) and a regional wellbore structure design coefficient. 3.The risk assessment-based design method for the deep complex formationwellbore structure according to claim 1, wherein the method forcalculating the risk coefficients of each layer of casing in (2) is asfollows: (2-1) probabilistic distribution of formation pressure theprediction error ΔP_(i) of the formation pressure P_(i) is a function ofthe well depth H:ΔP _(i) =f(H)ϵ[P _(i0) ,P _(i1)],  (1) in formula (1), P_(i0) is thelower limit value of the error, P_(i1) is the upper limit value of theerror, and i represents the type of the formation pressure, wherein theprobabilistic distribution of the prediction error of the formationpressure satisfies the following rule: $\begin{matrix}{{f\left( P_{i} \right)} = {\frac{1}{\sqrt{2\pi}\sigma_{p_{i}}}{\exp\left( {- \frac{\left( {P_{i} - \frac{P_{i1} + P_{i0}}{2}} \right)^{2}}{2\sigma_{p_{i}}^{2}}} \right)}}} & (2)\end{matrix}$ in formula (2), σ_(P) _(i) the standard deviation of P_(i)and is selected according to the prediction accuracy, and the valuerange is (0, 1); the cumulative probability corresponding to thepredicted value P_(i) of the formation pressure is: $\begin{matrix}{{P\left( P_{i} \right)} = {\int_{- \infty}^{P_{i}}{\frac{1}{\sigma_{p_{i}}\sqrt{2\pi}}e^{\frac{{({P_{i} - \frac{P_{i1} + P_{i0}}{2}})}^{2}}{2\sigma_{p_{i}}^{2}}}{dP}_{i}}}} & (3)\end{matrix}$ for the formation pore pressure, the prediction error isΔP_(p)ϵ[P_(p0), P_(p1)], for the formation fracture pressure, theprediction error is ΔP_(f)ϵ[P_(f0), P_(f1)]; (2-2) probabilisticdistribution of wellbore structure design coefficient if the value rangeof the wellbore structure design coefficient K is [K₀, K₁], then theprobabilistic distribution formula thereof is as follows:$\begin{matrix}{{f(K)} = {\frac{1}{\sqrt{2\pi}\sigma_{K}}{\exp\left( {- \frac{\left( {K - \frac{K_{1} + K_{0}}{2}} \right)^{2}}{2\sigma_{K}^{2}}} \right)}}} & (4)\end{matrix}$ in formula (4), σ_(K) is the standard deviation of K andis actually selected according to the drilling of a region where atarget well is located, and the value range is (0, 1); a credibility Jis set to obtain the distribution interval of each design coefficient Kas [f₀(K), f_(n)(K)]; in the distribution interval, the cumulativeprobability corresponding to the design coefficient f_(i)(K) is:$\begin{matrix}{{P\left( {f_{i}(K)} \right)} = {\int_{- \infty}^{f_{i}{(K)}}{\frac{1}{\sigma_{K}\sqrt{2\pi}}e^{- \frac{{({{f_{i}{(K)}} - \frac{{f_{i}{(K)}} + {f_{0}{(K)}}}{2}})}^{2}}{2\sigma_{K}^{2}}}{d\left( {f_{i}(K)} \right)}}}} & (5)\end{matrix}$ the distribution intervals of kick tolerance S_(k),formation fracture pressure safety factor S_(f), additional drillingfluid density Δρ and suction pressure factor S_(b) are respectivelyexpressed as: [f₀(S_(k)),f_(n)(S_(k))], [f₀(S_(f)), f_(n)(S_(f))],[f₀(Δρ), f_(n)(Δρ)] and [f₀(S_(b)),f_(n)(S_(b))]; (2-3) downholeengineering risk calculation for an Nth-layer casing at the well depth Hthe downhole engineering risk R(H) at the well depth H is calculatedaccording to the pressure balance relationship:kick risk: R _(JY)(H)=m[1−P(P _(p)(H))]×[1−P(f _(n)(S _(b)))]×[1−P(f_(n)(Δφ)]  (6) where, $m = \left\{ \begin{matrix}0 & {\rho_{m} > {{P_{p}(H)} + {f_{n}\left( S_{b} \right)} + {f_{n}\left( {\Delta \rho} \right)}}} \\1 & {\rho_{m} \leq {{P_{p}(H)} + {f_{n}\left( S_{b} \right)} + {f_{n}\left( {\Delta \rho} \right)}}}\end{matrix} \right.$risk of lost circulation: R _(JL)(H)=m×P(P _(f0)(H))×[1−P(f _(n)(S_(k)))]×[1−P(f _(n)(S _(f)))]  (7) where, $m = \left\{ \begin{matrix}0 & {\rho_{m} < {{{f_{n}\left( S_{k} \right)} \times \frac{H}{H_{n - 1}}} + {f_{n}\left( S_{f} \right)} + {P_{f0}(H)}}} \\1 & {\rho_{m} \geq {{{f_{n}\left( S_{k} \right)} \times \frac{H}{H_{n - 1}}} + {f_{n}\left( S_{f} \right)} + {P_{f0}(H)}}}\end{matrix} \right.$ in formulas (6) and (7), ρ_(m) is the equivalentdensity of drilling fluid, and H_(n-1) is the depth of the last casingshoe; (2-4) determination of risk coefficients of each layer of casingthe downhole engineering risks at the well depth H calculated in (2-3)are integrated within the range of the layer of casing to obtain theoverall risk coefficient R_(N) of the Nth-layer casingR _(N)=∫_(H) _(n) ^(H) ^(m) (R _(JY)(H)+R _(JL)(H)dH  (8) in formula(8), H_(n) is the minimum depth of the Nth-layer casing; and H_(m) isthe maximum depth of the Nth-layer casing.